The Group III nitride semiconductor has been being reduced to commercial products, such as LEDs and LDs, because it possesses a direct transition-type band gap of energy corresponding to a region extending from visible light to ultraviolet light and allows highly efficient luminescence. It further has the potential for acquiring such characteristic properties of an electronic device as the conventional Group III-V compound semiconductor fails to acquire, as evinced by forming in the heterojunction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN) a two-dimensional electron layer due to a piezoelectric effect characteristic of a Group III nitride semiconductor.
A large lattice mismatching exists between the material generally used as the substrate for growing a Group III nitride semiconductor and a Group III nitride semiconductor. For example, a lattice mismatching of 16% exists between sapphire (Al2O3) and gallium nitride (GaN) and a lattice mismatching of 6% exists between SiC and gallium nitride. Generally, when such a large lattice mismatching as these exists, it is difficult to attain epitaxial growth of a crystal directly on a substrate. Even when the growth is attained, a crystal of excellent crystallinity is not obtained. When the Group III nitride semiconductor crystal is epitaxially grown on a sapphire single crystal substrate or a SiC single crystal substrate by the method of metal organic chemical vapor deposition (MOCVD), therefore, a method which comprises first depositing on a substrate a layer formed of aluminum nitride (AlN) or AlGaN and called a low-temperature buffer layer and epitaxially growing thereon a Group III nitride semiconductor crystal at an elevated temperature has been generally adopted (refer to Japanese Patent No. 3026087 and JP-A HEI 4-297023).
Besides the method of growth adopting the low-temperature buffer layer mentioned above, a method which comprises forming on a substrate an AlN layer grown at an elevated temperature in the approximate range of 900° C. to 1200° C. and growing gallium nitride thereon has been disclosed (refer, for example, to JP-A HEI 9-64477 and P. Kung, et al., Applied Physics Letters, 66 (1995), p. 2958).
Further, a method of fabricating a Group III nitride semiconductor crystal layer stacked structure using as a buffer a Group III nitride semiconductor fabricated using a Group V/III element ratio of 1000 or less has been disclosed (refer to JP-A 2003-243302).
At 400° C. to 600° C. as temperatures for the deposition of a low-temperature buffer layer, the organic metal raw material or the nitrogen source, particularly ammonia used as a nitrogen source, undergoes no sufficient thermal decomposition. The low-temperature buffer layer deposited at such low temperatures contains defects copiously in its unaltered form. Since the raw material is subjected to a reaction at a low temperature, the reaction entails polymerization as between the alkyl group of the organic metal of the raw material and the undecomposed nitrogen source, and the impurity resulting from this reaction is copiously contained in the crystal of the low-temperature buffer layer.
It is the process of heat treatment called the crystallization of a low-temperature buffer layer that is resorted to for the elimination of such defects and impurity. The process for crystallizing the buffer layer effects the removal of the impurity and defects copiously contained in the low-temperature buffer layer by subjecting this low-temperature buffer layer to a heat treatment performed at elevated temperatures approximating closely to the temperature for the epitaxial growth of the Group III nitride semiconductor crystal.
In contrast with the method of growth using this low-temperature buffer layer, the method which, as disclosed in “P. Kung, et al., Applied Physics Letters, 66 (1995), p. 2958” mentioned above, comprises forming on a substrate AlN grown at elevated temperatures in the approximate range of 900° C. to 1200° C. and then growing gallium nitride thereon has been available. This prior art contains a mention to the effect that this method is capable of fabricating a veritably excellent crystal describing an X-ray locking curve of 30 arcsec on the (0002) plane. A double check of this process has resulted in yielding a gallium nitride crystal film formed of a crystal having a very high column forming property and containing numerous grain boundaries. The crystal of this quality contains threading dislocations occurring from the substrate toward the surface at a high density. When this product is fabricated into a device configuration of a light-emitting device or an electronic device, therefore, the device does not acquire satisfactory characteristic properties.
A method for growth which uses an AlN layer fabricated similarly at elevated temperatures is also disclosed in JP-A HEI 9-64477. As described in this document, the Group III nitride semiconductor crystal to be fabricated is preferred to be a single crystal excelling in crystallinity. In spite of a repeated double check, the method for growth using an excellent single crystal AlN film as described in this prior art, similarly to the method described in the preceding prior art, has not been found to grow such a crystal as fabricating a device structure and acquiring ideal characteristic properties. This failure may be logically explained by a supposition that when the layer of a single crystal excelling in crystallinity is used as a buffer layer and a Group III nitride semiconductor is subsequently grown on the buffer layer, the atoms adhering to the buffer layer during the initial stage of the growth are not smoothly migrated and are not allowed to attain two-dimensional growth easily.
Since the Group III nitride semiconductor crystal possessing crystallinity sufficient to fabricate a device cannot be obtained as described above, the method for growing a Group III nitride semiconductor crystal using an AlN buffer layer grown at elevated temperatures is not quite popular at present.
The technique of forming an AlN film under the condition of restricting the V/III ratio to 1000 or less as disclosed in JP-A 2003-243302 is indeed capable of suppressing an electric power consumed and allaying the warping of the substrate and nevertheless entails the problem that the GaN formed thereby on the AlN film is deficient in crystallinity.
This invention is aimed at developing a method for fabricating a Group III nitride semiconductor crystal, particularly a GaN crystal, exhibiting further excellent crystallinity based on the technique disclosed in JP-A 2003-243302, namely at providing a method for fabricating the crystal of a Group III nitride semiconductor, such as GaN, of excellent crystallinity without requiring to set many temperature ranges or necessitating an excess electric power.